Yeah, Megadeth have had a very uneven career. I think all the best albums are the ones with Marty Friedman on guitar, but I don't think the Marty Friedman albums are the only good ones. With Mustaine I just think Friedman is the secret sauce.
ALL of Megadeth's best selling albums "Rust In Peace," "Countdown To Extinction" and "Youthanasia" have Friedman on them and I do NOT think that's a happy coincidence, I think he brings something to the table that Mustain lacks, specifically, the ability to play a coherent, melodic and interesting guitar solo.